1. Field of the Invention
The present invention relates to an image blur prevention apparatus for preventing image blur generated in a camera, an optical apparatus (device) or the like.
2. Related Background Art
In recent cameras, the probability of failure in the phototaking operation is very low even for an unskilled person for the camera operation, because the important operations for phototaking such as determination of exposure and focusing are all automated.
Also there has been developed an image blur prevention system for avoiding hand fluctuation applied to the camera, and almost no factor is left to induce the error of the photographer in phototaking. Such image blur prevention system will be explained briefly.
The hand fluctuation applied to the camera in phototaking operation is generally a vibration of a frequency of 1 to 12 Hz. The basic concept for obtaining a photograph without image blur even in the presence of such hand fluctuation when the shutter is released is to detect the fluctuation of the camera induced by such hand fluctuation and to displace a correction lens according to the detected value.
More specifically, in order to obtain a photograph without image blur even in the presence of camera fluctuation, it is essential to first detect the fluctuation of the camera exactly, and then to correct the variation in the optical axis resulting from the hand fluctuation. The detection of such camera fluctuation is achieved, in principle, by providing the camera with fluctuation detection means for detecting the angular acceleration, angular velocity or angular displacement, and camera fluctuation detection means for releasing an angular displacement by integrating the output signal of such fluctuation detection means in electrical or mechanical manner. The image blur prevention is achieved by displacing optical support means (correction means), supporting an optical element such as a lens or a prism, in a direction perpendicular to the optical axis, based on the detection information.
FIG. 19 is a schematic view of a fluctuation prevention system employing conventional fluctuation detection means, utilized for example in a camera, for suppressing the image blur in directions indicated by arrows 81 (camera pitching 81p, and camera yawing 81y).
In FIG. 19 there are shown a lens barrel 82;
fluctuation detection means 83p, 83y for respectively detecting the camera pitching (in a direction 84p) and camera yawing (in a direction 84y); correction means 85, supporting a correcting optical element (such as prism or lens), for correcting the image blur resulting from the fluctuation; coils 86p, 86y for providing the correction means 85 with a driving force; and position detectors 87p, 87y for detecting the position of the correction means 85. The correction means 85 functions utilizing a position control loop and using the output signals from the fluctuation detection means 83p, 83y as target values, thereby correcting the image blur resulting from the fluctuation.
FIG. 20 is a schematic block diagram of a conventional image blur correction system, wherein an output signal from fluctuation detection means 2 is amplified by amplifier means 3, and supplied to an A/D converting port of a microcomputer 1. An output signal from position detection means 4, for detecting the position of the correction lens, is amplified by amplifier means 5, and supplied to an A/D converting port of the microcomputer 1. The microcomputer 1 processes these two data to release correction lens driving data from an output port, thereby causing correction lens driving means 6 to drive the correction lens, thus correcting the image blur. Lock/unlock drive means 7 executes drive of an unlock coil and maintaining of an unlocked state for locking or unlocking the drive for the correction lens.
FIGS. 21A and 21B are flow charts showing the function of the microcomputer 1 shown in FIG. 20. The image blur correction is executed by an interruption process, conducted for example at a predetermined interval. The main flow executes for example the lock/unlock control. When an interruption process is generated, the sequence starts from a step #81.
Step #81! executes A/D conversion of the output of an angular velocity sensor, constituting the fluctuation detection means.
Step #82! discriminates whether an instruction for initiating the image blur correction has been received, and the sequence proceeds to a step #85 or #83, respectively, if the instruction has been received or not.
Steps #83 and #84 are operations when the image blur correction is not conducted.
Step #83! initializes the DC offset and the integration, as the image blur correction is not executed.
Step #84! clears a timer for measuring the time after the reception of the image blur correction starting instruction.
Step #85! discriminates whether a predetermined time has elapsed since the reception of the image blur correction starting instruction. This corresponds to an operation time for determining the DC offset in a step #86, so that the image blur correction is not yet executed. The sequence proceeds to a step #88 or #86, respectively if the predetermined time has elapsed or not.
Steps #86 and #87 are operations within the predetermined time after the reception of the image blur correction starting instruction.
Step #86! calculates the DC offset, in order to avoid that the initial input to the high-pass filter becomes a stepped input because of the DC component.
Step #87! initializes the high-pass filter and sets the result of integration to zero, in order to electrically maintain the correction lens at the central position.
Steps starting from #88 execute the image blur correction.
Step #88! effects a high-pass filter operation, in order to effect the image blur correction.
Step #89! executes integration, to provide angular displacement data.
Step #90! adjusts the eccentricity (sensitivity) of the correction lens with respect to the fluctuation angular displacement, since it varies depending on the zoom/focus position.
Step #91! stores the result of calculation (drive data for image blur correction) in a RAM area, set by SFTDRV in the microcomputer 1.
Step #92! effects A/D conversion of the output from a position sensor, detecting the position of the correction lens, and stores the result in SFTPST in the RAM.
Step #93! effects feedback calculation (SFTDRV-SFTPST).
Step #94! multiplies the result of the calculation in the step #93 with a loop gain.
Step #95! effects a phase compensating calculation, for obtaining a stable control system.
Step #96! sends the result of the step #95, as PWM, to the port of the microcomputer, whereupon the interruption process is terminated.
The obtained output is supplied to a coil driver for driving the correction lens, thereby driving the correction lens with a moving coil to correct the image blur.
The image blur correction is achieved in the configuration and the system explained above.
The output signal of the angular velocity sensor, constituting the fluctuation detection means, is the sum of a DC component and a hand fluctuation component, as indicated by a solid line 10S1 in FIG. 22A. Consequently, the step #85 in FIG. 21A effects averaging by the application of a low-pass filter, thereby reducing the hand fluctuation component and determining the DC offset. Then the value of a broken line 10S2, after the lapse of a predetermined time, is regarded as the DC offset, and the value obtained by subtracting the DC offset from the A/D converted value of the output of the angular velocity sensor is used as the input data for the high-pass filter calculation.
In the ordinary hand-held state of the camera, the error between the actual DC offset and the above-determined DC offset (different between a DC offset signal 10R1 in FIG. 22A, indicating the actual DC offset, and the broken line 10S2 obtained with the low-pass filter) is small and negligible. However, if the image blur correction is started while the camera is swung in a large motion, the angular velocity signal assumes a form as indicated by a solid line 10S3 in FIG. 22B and can only provide a broken-lined signal 10S4 even after averaging with a low-pass filter, thus resulting in a significant error from the actual DC offset indicated by 10R2.
Such signal results in a significant transient response by the stepped input in the high-pass filter operation, deteriorating the start-up characteristics of the image blur correction.